1. Field of the Invention
The present invention relates to roentgenoscopy and, more particularly, to medical X-ray diagnostic and so called X-ray flaw detection or anti-scatter X-ray rasters (grids), which filter a radiation, transmitted through an object toward a detecting device, and absorb a Compton secondary radiation.
2. Description of the Prior Art
X-rays experience an increase in wavelength due to a reduction in energy after being scattered through an angle. This phenomena is referred to a the Compton scattering or Compton secondary radiation. This Compton secondary radiation occurs when X-raying the internal structure of objects in medicine and industry and is the main factor which decreases contrast and degrades the quality of X-ray images.
A high probability of forming quanta of a secondary radiation is characteristic of the Compton effect. That is, there can be several photons of a secondary scattered radiation for every photon of a radiation falling on the object under study which is formed by a primary X-ray source. This significantly reduces quality of the resultant image because of the diffuse nature of the secondary radiation. A purpose of the present invention is to eliminate photons of the secondary Compton radiation, which are moving in a different direction as photons of the primary radiation source.
This problem could be solved by using an anti-scatter raster, having the ability to pass through the photons of the primary radiation as well as that photons of a secondary Compton radiation, which are moving is a direction approximately the same as a primary photons.
Prior art anti-scatter rasters comprise a grid, placed between the object under study and the means for detecting. The grid comprises strips made of a material which can absorb the secondary Compton radiation. The length of strips exceed the distance between them by several times. (see, for example, Chan H.-P., Frank P. H., Doi K., Iida N., Higashida Y. Ultra-High-Strip-density Radiographic Grids: A New Anti-scatter Technique for Mammography. xe2x80x9cRadiologyxe2x80x9d, volume 154, Number 3, March 1985, pp. 807-815).
The strips of the grid are generally oriented in the direction parallel to the direction of spreading of the primary radiation. It means that this primary radiation easily passes through the grid. In contrast the Compton secondary radiation is absorbed by the grid. Intensity of the primary radiation is slightly reduced because of the limited transparency of the grid due to the thickness of the strips.
A level of suppression of the secondary Compton radiation is defined by the relationship between a size of the strips in the direction of spreading primary radiation to a distance between neighboring strips. This parameter, called the aspect relationship, defines an value of the angle of deviation of photons of secondary radiation from direction of spreading photons of primary radiation. The greater the aspect relationship, the less is the angle, and subsequently the less the amount of photons of the secondary radiation reach a detecting device, which improves the contrastive quality of an image in the center. However, the brightness and contrast of a peripheral or edges of the image are decreased.
To avoid this negative effect, a grid can be made focused, which means the strips are not parallel to each other, but have an increasing radial angle from the center of the grid to the edges of the grid. The strips are placed in such a way that they provide a parallelism of their planes to the direction of photons of passed through the object under study. In a focused grid, the planes of all strips are oriented to point to the source of a primary radiation, which ideally may be represented as a point source. Thus radiation emanating from the point source spreads parallel with the axial direction of the strips. (see for example, Physics of image visualizing in medicine. Ed. by S. Webb. Moscow, xe2x80x9cMirxe2x80x9d, 1991, volume 1, pp. 131-133).
Despite the fact that the above mentioned texts describe the usage of an anti-scatter grid only for medical purposes, the general concept of eliminating a secondary radiation is applicable as well for any X-ray applications including internal industrial structures or non-biological objects.
Manufacturing of an anti-scatter raster faces the problem of placing thin strips made of absorbing material a small distance from each other with the precision of their plane orientation. Moreover, the strips have a thickness associated with them. Thus the cross-sectional area of the strips due to the strip thickness has the negative effect of also decreasing the transparency of a grid for a primary radiation. This diminishing of transparency could be compensated for by increasing of intensity of a primary radiation or increasing exposure time. However, both of these measures are undesirable because it increases the radiation dose, which is particularly bad if the object under study is a human patient.
Using thinner strips would of course allow more primary radiation to pass. However, using thinner strips in the grid causes trouble since thinner strips are not as structurally robust. Thus, spacers made of an X-ray transparent material should be placed between absorbing strips to provide necessary stiffness and mechanical strength. However, spacers are problematic in the case of a focused grid since the spacers are required to have a varying thickness along a direction of spreading of radiation (i.e. they must be tapered).
The usage of spacers causes additional losses of intensity of a radiation passed to the detecting device. This in turn causes the above said negative impacts, which require increasing the power of a primary radiation or exposure time. This of course increases the irradiation dose to the object under study as well as maintenance personnel. The Bucky factor characterizes this increase, and according the data of a source Chan et al., supra, it is 2 to 8, and in source Webb, supra, the values are in limits of 2 to 4.5.
According to source Chan et al. actual obtainable distance between the strips is on the order of 150 xcexcm, and according to source Webb it is between 67 to 150 xcexcm.
In the prior art, if an anti-scatter raster is made of strips, the channels for transporting the primary X-rays passed through an object are slots. As a result the secondary radiation which is traveling parallel to the walls of slots (i.e. parallel to the strips) or close to them, is not suppressed.
The present invention overcomes the disadvantages of prior art by increasing the aspect relationship without deteriorating the transparency of an anti-scatter raster for the primary radiation. The invention selects photons of secondary radiation which have an angle exceeding an angle determined by an aspect relationship and deviates from the direction of a primary radiation in any plane. Therefor satisfactory attenuation of intensity of a secondary radiation on the detecting device can obtain even with small aspect relationship. It allows obtaining acceptable results using a non-focused raster, which is easier to produce and does not require placement at a well-defined distance from the source.
The suggested anti-scatter X-ray raster is disclosed in three embodiments, the first one being a focused anti-scatter raster, and the second and third embodiments being of the focused type. A common feature to all three embodiments is a great number of channels for X-rays transporting, and channels of the walls being made of an X-ray absorbing material.
According to the invention, rather than using strips resulting in a slotted structure as in the prior art, the present invention uses a cellular structure realized by using a plurality of tubular channels for X-ray transporting having adjoining walls spliced together. Thus the largest cross size of a single channel (d), and its length (H) satisfies a relationship 2d/H greater than xcex8c, where xcex8c is a critical angle of total external reflection of an X-rays from the walls of the channels.
In all three embodiments the walls of tubular channels are made of a material, capable of absorbing X-ray, for instance, of lead glass. They can be made as well of dielectric or metal. Thus the channels can be both hollow and filled with light metal or organic material. The channels can be formed as well by glass mono or polycapillaries.
According to the first embodiment (non-focused raster) the channels have parallel longitudinal axes, and according to the second and third embodiments, (focused raster) the channels have longitudinal axes, which continuations outside a device converge in one point, a focus, where an X-rays source must be when an anti-scatter raster is in usage.
According to the first embodiment the tubular channels for an X-rays transporting can have a form of a cylindrical surface (need not be circular) or a form of a side surface of a prism (need not be regular). The open ends, (i.e., the inputs and outputs of the channels) according to the first embodiment are placed, are parallel planes perpendicular to the longitudinal axes of all channels. The second embodiment differ in that the open ends are placed on different planes having concentrical spheres with a center in a focus (i.e. the center point being the point where continuations of longitudinal axes of the channels would converge).
According to the third embodiment these surfaces (the planes of the inputs and outputs) are planes parallel to each other, being perpendicular to a longitudinal axis of one of the channels, placed in the central zone of an anti-scatter raster.
According to the second as well as to the third embodiment the walls of the channels can be in a form of side surface of a truncated cone or a truncated pyramid, thus the tops of the figures for all channels coincide with a focus. The cones do not need to be circular, and pyramids do not need to be regular.
Besides the above, other specific cases of embodiment of an anti-scatter raster, related to all three variants, are possible.
According to one specific way an anti-scatter raster in cross-section, corresponding to the direction of an longitudinal axis of one of its channels placed in the central zone of a raster (for the raster according to the first embodiment these directions are equal for all channels), has a size, corresponding to the full size of the object under study or to the full size of a part of the object, and in this projection, in particular, it can have a form of rectangle with comparable sizes of its sides.
According to the other specific ways an anti-scatter raster is intended for usage in a scanning system, when a narrow zone of the object under study is irradiated through a slot collimator, being moved with respect to an object, and an anti-scatter raster, placed on the other side of the object under study just opposite a slot of a collimator, moves in synchronism with the slot.
In this case, the anti-scatter raster can be in the form of narrow brick (i.e., box shape) with a great quantity of cells in one direction, and a smaller amount of cells in other direction, being parallel to a slot of a collimator, and in the other direction, being perpendicular to the first direction (i.e. in the direction of moving a slot of a collimator).